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Why You Have To Scratch That Itch

Everybody itches. Sometimes itch serves as a useful warning signal — there's a bug on your back! But sometimes itch arises for no apparent reason, and can be a torment.

Think of the itchy skin disorder eczema, or the constant itching caused by some cancers. "A very high percentage of people who're on dialysis for chronic kidney disease develop severe itch that's very difficult to manage," says Dr. Ethan Lerner, an associate professor of dermatology at Harvard Medical School.

Scientists now say they've got a much better clue as to how itch happens.

For a long time, the thought was that itch piggybacked on the nerves that feel pain or temperature. But it now looks like itch has its own dedicated highway from skin to brain.

And the molecule that makes itch happen comes as a surprise; it usually hangs out in the heart, where it helps control blood pressure.

It's a neurotransmitter called natriuretic polypeptide B, or Nppb.

Researchers at the National Institute of Dental and Craniofacial Research wondered what it was doing in nerve cells. To find out, they created a mouse that didn't make Nppb in its body.

Things that made a normal mouse scratch like crazy had no effect on mice with no Nppb. But when those mice were injected with the substance, they scratched, too.

Nppb seems to be working sort of like an itch-molecule. Take it away and the mice don't itch. Put it back and the itch returns.

The researchers also found a small group of nerves in skin that produce and use this molecule to send an itch message to the spinal cord.

This research hasn't been replicated in humans, so it doesn't prove that human itch works the same way. But the researchers are confident that the molecule is a key clue in defining the long-elusive itch pathway.

An old saying has it that an itchy nose means you're destined to kiss a fool. Another claims if your palm itches, you're going to come into some money. A study released this week sheds a more scientific light on itching, and gives one explanation as to what goes on in our bodies when we have that irresistible urge to scratch.

NPR's Rhitu Chatterjee has the story.

RHITU CHATTERJEE, BYLINE: Ethan Lerner is a dermatologist at Harvard Medical School. He's spent his career trying to understand how and why we itch. Lerner says itching evolved as a way to protect us, to make us scratch.

ETHAN LERNER: To remove bugs of various types that are trying to feed off of us or burrow into us.

CHATTERJEE: But sometimes the itching seems to have no apparent purpose. Lerner says think of eczema or the constant itching you get with some cancers and in chronic kidney disease.

LERNER: A very high percentage of people who are on dialysis have intense itch, which is very difficult to manage.

CHATTERJEE: He says scientists know very little about what's actually going on inside our bodies when we itch. It's a complex circuit that links the skin to the brain.

LERNER: If we view itch as a puzzle, there are many components. These include the skin, the immune cells in the skin, the sensory nerves that extend from the skin to the spinal cord.

CHATTERJEE: Lerner says these nerve fibers that are just below our skin sense all sorts of changes in our environment. If someone turns up air conditioning, you feel cold. It hurts if you prick yourself. Now, scientists have long thought that itching probably involved the same nerves.

MARK HOON: We were particularly interested in a set of neurons that are responsible for the detection of not only itch, but also temperature and some types of pain.

CHATTERJEE: That's neuroscientist Mark Hoon. He's at the National Institutes of Health outside Washington and one of the authors of the study. He and his team decided to look more closely at what these nerve cells were doing. They started working with lab mice. Soon things got really interesting.

They discovered that some of the nerve cells they were looking at were producing an unexpected molecule.

HOON: Natiureitc polypeptide B. It's a very long name.

CHATTERJEE: NPPB for short. It's found in the heart and kidneys. But Mark Hoon had no idea what it was doing in these nerve cells. So he used genetic engineering to create a mouse that didn't make NPPB, and watch what happened.

HOON: We tested it for hot, for cold, for pain.

CHATTERJEE: All normal.

HOON: And we tested it also for itch.

CHATTERJEE: The result was surprising. Things that made a normal mouse burst into frenzied bouts of itching had little effect on the mouse with no NPPB. Parts of the itch puzzle had begun to fall into place. NPPB seems to be a specific itch molecule. And the small group of nerve fibers producing it are a special itch highway between the skin and the spinal cord.

Mark Hoon and his team report their findings in this week's Science magazine. When Ethan Lerner, the Harvard dermatologist, read the study, he was excited.

LERNER: I would say it's fairly big.

CHATTERJEE: Now, it's difficult to tell as yet whether this works in the same way in people. But if it does, Lerner says blocking this molecule, or the nerves producing it, could be a way to help patients with acute and chronic itching.

In the meantime...

LERNER: I never tell a patient not to scratch an itch. If you have an itch, you need to scratch it.

CHATTERJEE: Rhitu Chatterjee, NPR News.

MONTAGNE: And you can see a video of the itchy and the non-itchy mouse at NPR.org.

(SOUNDBITE OF SONG, "SCRATCH, SCRATCH ME BACK")

CHORUS: (Singing) Scratch, scratch me like. Scratch, scratch me back. It really is a fact that if I itch the more I scratch.